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Douglas Carswell, elected in 2010 as Conservative MP for Clacton, has today announced he’s joining Ukip and will fight a by-election under his new party’s banner.

In one sense, the news isn’t a surprise. Carswell is a member of the Tory awkward squad, its sixth most rebellious backbencher according to Revolts.co.uk, having defied the party whip on 46 occasions during this Parliament.

But on another level it’ll be a real shock to the Tories: Carswell’s right-wing brand is much less swivel-eyed than that of many of his fellow rebels like Philip Hollobone and David Nuttall. He’s generally a thoughtful, independent-minded politician, as he showed with his most recent book The End of Democracy. He is not one of the classic ‘fruitcakes, loonies and closet racists’ as David Cameron once famously dismissed Ukip supporters.

More importantly he has a very good chance of winning the by-election – he has a majority of over 12,000 and Clacton’s demographics are very Ukip-friendly – thereby giving Nigel Farage’s party its first elected MP. (Bob Spink was Ukip’s first ever MP, but not elected as such.) That would give a further injection to the Ukip publicity machine and give them a better chance of getting on the podium of the televised leaders’ debates. There’s also the hint this isn’t the only Tory-to-Ukip defection we may see in what remains of this parliament.

Coupled with today’s net immigration figures – up from 175,000 to 243,000, leaving the Tory pledge to reduce it to tens of thousands in tatters – and it’s been a good one for Ukip and pretty terrible for the Tories.